1. Field of the Invention
The present invention relates to an electrophotographic apparatus and an electrophotographic photosensitive member.
2. Description of the Related Art
An electrophotographic photosensitive member is employed in various steps, such as charging, image exposure, development, transferring, and cleaning, so the surface of the electrophotographic photosensitive member is worn with use. To address this, a technique for providing an electrophotographic photosensitive member with a surface layer resistant to wearing in order to enable the electrophotographic photosensitive member to withstand long term use has become practical. However, even if such a surface layer resistant to wearing is provided, wearing still exists and the surface layer is gradually worn by long use.
For example, in the case of an electrophotographic photosensitive member that includes a photoconductive layer made of amorphous silicon, a technique for providing a surface layer made of amorphous silicon carbide on the photoconductive layer has become practical. As in this case, if the photoconductive layer and the surface layer are made of different materials, because the materials have different refractive indices, part of an image exposure beam is reflected at the interface between the photoconductive layer and the surface layer. For the same reason, part of the image exposure beam is also reflected at the interface between the surface layer and the air. These two reflected beams interfere with each other, and the interference conditions are chiefly determined by the refractive index and thickness of the surface layer. As a result, if the surface layer is worn with use, the interference conditions vary, the light quantity of the image exposure beam reaching the photoconductive layer inevitably changes, and the sensitivity of the electrophotographic photosensitive member varies.
Here, reflection occurring at interfaces between multiple films in which layers of different refractive indices are laminated is described.
When a beam impinges on an interface between two layers of different refractive indices, part of the incident beam is reflected at the interface. Specifically, as illustrated in FIG. 4A, when a beam impinges on a layer of refractive index n2 at an angle of incidence θ1 from a layer of refractive index n1, amplitude reflectance r and amplitude transmittance t can be represented from Fresnel equations by the following expressions (12) to (15), where the angle of refraction is θ2.
For an S wave, in which a plane of incidence is perpendicular to a plane of polarization:
                    r        =                                                                              n                  1                                ·                cos                            ⁢                                                          ⁢                              θ                1                                      -                                                            n                  2                                ·                cos                            ⁢                                                          ⁢                              θ                2                                                                                                          n                  1                                ·                cos                            ⁢                                                          ⁢                              θ                1                                      +                                                            n                  2                                ·                cos                            ⁢                                                          ⁢                              θ                2                                                                        (        12        )                                t        =                                            2              ·                              n                1                            ·              cos                        ⁢                                                  ⁢                          θ              1                                                                                            n                  1                                ·                cos                            ⁢                                                          ⁢                              θ                1                                      +                                                            n                  2                                ·                cos                            ⁢                                                          ⁢                              θ                2                                                                        (        13        )            
For a P wave, in which a plane of incidence is parallel to a plane of polarization:
                    r        =                                                                              n                  1                                /                cos                            ⁢                                                          ⁢                              θ                1                                      -                                                            n                  2                                /                cos                            ⁢                                                          ⁢                              θ                2                                                                                                          n                  1                                /                cos                            ⁢                                                          ⁢                              θ                1                                      +                                                            n                  2                                /                cos                            ⁢                                                          ⁢                              θ                2                                                                        (        14        )                                t        =                                            2              ·                                                n                  1                                /                cos                                      ⁢                                                  ⁢                          θ              1                                                                                            n                  1                                /                cos                            ⁢                                                          ⁢                              θ                1                                      +                                                            n                  2                                /                cos                            ⁢                                                          ⁢                              θ                2                                                                        (        15        )            
From Snell's law, the angle of incidence θ1 and the angle of refraction θ2 satisfy the following expression (16):
                                          sin            ⁢                                                  ⁢                          θ              1                                            sin            ⁢                                                  ⁢                          θ              2                                      =                              n            2                                n            1                                              (        16        )            
An electrophotographic apparatus typically exposes an electrophotographic photosensitive member with an image exposure beam for forming a latent image on the surface of the electrophotographic photosensitive member at an angle nearly perpendicular thereto. Specifically, typical angles of incidence in exposure are approximately ±15° in a main scanning direction and approximately 5° or less in a sub scanning direction. A typical refractive index of a material used in the surface layer of the electrophotographic photosensitive member is 1.5 or more. If amorphous silicon carbide is used as the material of the surface layer, because the refractive index is 1.9 or more, a beam passing through the surface layer is incident on a lower layer at an angle less than 10°. Accordingly, when reflection at an intermediate layer between the surface layer and the photoconductive layer is considered, no great problem occurs if θ1=θ2≈0. From this approximation, the amplitude reflectance r and the amplitude transmittance t can be represented by the following expressions (17) and (18):
                    r        =                                            n              1                        -                          n              2                                                          n              1                        +                          n              2                                                          (        17        )                                t        =                              2            ·                          n              1                                                          n              1                        +                          n              2                                                          (        18        )            
The reflected beam intensity R is |r|2, and the transmitted beam intensity T is 1−R.
From the foregoing, it is revealed that the reflected beam intensity at an interface is determined by the refractive indices of two materials of media of the interface. When the amplitude reflectance r is positive, the phase of an incident beam and that of a reflected beam match with each other; when the amplitude reflectance r is negative, the phase of an incident beam and that of a reflected beam are shifted by π. Accordingly, when a beam impinges on a high-refractive-index layer from a low-refractive-index layer, the phase difference between a reflected beam and an incident beam is π; when a beam impinges on a low-refractive-index layer from a high-refractive-index layer, the phase difference between a reflected beam and an incident beam is 0.
There is a known technique of providing an antireflective layer between two layers of different refractive indices to reduce reflection of a beam occurring at the interface between the two layers. For example, as illustrated in FIG. 4B, if a single antireflective layer is disposed between a layer of refractive index n1 and a layer of refractive index n2, reflection of an incident beam of wavelength λ can be prevented when the refractive index n3 and the thickness d3 of the antireflective layer satisfy the following expressions (19) and (20), respectively:
                              n          3                =                                            n              1                        ·                          n              2                                                          (        19        )                                          d          3                =                  λ                      4            ·                          n              3                                                          (        20        )            
Under the above conditions, a reflected beam at an interface A between the layer of refractive index n1 and the antireflective layer of refractive index n3 and a reflected beam at an interface B between the antireflective layer of refractive index n3 and the layer of refractive index n2 cancel each other out, the interfaces being produced by the provision of the antireflective layer of refractive index n3. The amplitude reflectance when a beam incident from a direction substantially perpendicular to an interface is reflected at the interface can be calculated from the above expression (17). Therefore, the amplitude reflectance rA at the interface A and the amplitude reflectance rB at the interface B can be calculated from the following expressions (21) and (22):
                              r          A                =                                            n              1                        -                          n              3                                                          n              1                        +                          n              3                                                          (        21        )                                          r          B                =                                            n              3                        -                          n              2                                                          n              3                        +                          n              2                                                          (        22        )            
When the antireflective layer of refractive index n3 satisfies the above-described thickness condition, the phase difference between the reflected beam at the interface A and that at the interface B is π because of the difference in optical past length. Accordingly, if the magnitudes of rA and rB are equal, because rA and rB are cancelled out, a combined reflected beam is 0.
When the above expressions (21) and (22) are substituted into rA=rB, it is found that the refractive index n3 satisfies the above expression (19).
Japanese Patent Laid-Open No. 62-40468 discloses an electrophotographic photosensitive member that includes an antireflective layer for use in suppressing a variation in sensitivity of the electrophotographic photosensitive member.
The provision of an antireflective layer between a surface layer and a photoconductive layer can suppress a reflected beam between the surface layer and the photoconductive layer, prevent interference with a reflected beam at the interface between the surface layer and the air, and suppress a variation in sensitivity of the electrophotographic photosensitive member even if the surface layer is worn. Japanese Patent Laid-Open No. 62-40468 discloses an antireflective layer having a refractive index and a thickness that satisfy the above expressions (19) and (20), respectively, and also discloses an example in which the antireflective layer has a three-layer structure.
Japanese Patent Laid-Open No. 4-355403 discloses, as an example antireflective layer having a three-layer structure, an antireflective layer consisting of a first low-refractive-index layer, a second high-refractive-index layer, and a third low-refractive-index layer arranged in this order from the substrate side.
As in the related art, if an antireflective layer whose refractive index and thickness are optimized is provided between a surface layer and a photoconductive layer, reflection at the interface between the surface layer and the photoconductive layer can be suppressed. As a result, a variation in sensitivity of the electrophotographic photosensitive member to an image exposure beam having a predetermined wavelength can be suppressed.
However, a semiconductor laser frequently used as a light source for an image exposure beam in an actual electrophotographic apparatus often has a half-width of approximately plus or minus several nanometers with respect to a central oscillation wavelength, and a light-emitting diode (LED) often has a half-width of approximately 20 nm. It also has been known that an oscillation wavelength of a semiconductor laser has a temperature dependence of approximately 0.2 nm/° C. (e.g., 10 nm for a difference of 50° C.). Accordingly, a variation in sensitivity of an electrophotographic photosensitive member to an image exposure beam having a wavelength in a range from approximately 10 nanometers to several tens of nanometers is suppressed. In the related art, for a wavelength in such a wide range, the antireflection function may be insufficient, and a narrow allowable range for a wavelength of an image exposure beam is an issue.